Cathode for electrochemical regeneration of permanganate etching solutions

ABSTRACT

In order to regenerate permanganate solutions being utilized for the etching and roughening of plastics surfaces electrolytic methods are known. Though relatively small quantaties of by-products are produced with these methods as compared to chemical regeneration methods, large quantaties of manganese dioxide are produced when printed circuit boards are treated. In order to avoid formation of manganese dioxide during the regeneration method a novel cathode  2  has been found which is provided with a porous, electrically nonconducting layer  7  on the cathode body  3 . The layer  7  preferably consists of a plastics material being resistant to acid and/or alkali.

FIELD OF THE INVENTION

[0001] The invention relates to a cathode for an electrochemicalarrangement for regeneration of permanganate etching solutions, a methodfor the manufacture thereof and a device for electrochemicallyregenerating permanganate etching solutions.

BACKGROUND OF THE INVENTION

[0002] Alkaline permanganate solutions are utilized for etching plasticsmaterial, for example in the manufacture of printed circuit boards butalso prior to metallization of machined parts made of plastics forsanitary appliances, for the automotive industry, for precisionmechanics industry or for furniture mountings. These solutions usuallycontain 30 to 100 g alkali permanganate and 30 to 100 g alkali hydroxideper 1 liter aqueous solution. The potassium permanganate being lessexpensive than sodium permanganate is normally used at a concentrationbelow about 65 g/l and the considerably better soluble sodiumpermanganate is used at a higher concentration than 65 g/l. Typicallypotassium and sodium hydroxide are used as alkali hydroxide source. Ingeneral these solutions are operated at a temperature of from 60° C. to98° C. Normally the plastics surfaces are treated with an organicswelling agent prior to the actual etching process in order tofacilitate etching.

[0003] Due to the etching treatment of the plastics surfacespermanganate species are primarily reduced to manganate species, themanganate species on their part being disproportionated to permanganatespecies and to manganese dioxide. During this reaction considerableamounts of manganese dioxide sludge may be produced in the etchingsolution. A continuous operation of the method is not possible due toconsumption of permanganate. Moreover the method is very expensive sincepermanganate salts must be added to the solution and waste material mustbe disposed of permanently.

[0004] In order to solve these problems a regenerating method has beendevelopped. This method is described in EP 0 204 399 A1. In this methodan oxidizing agent is added to the spent permanganate etching solutionin an amount which is sufficient for reoxidizing manganate species inthe etching solution with manganese compounds having an oxidation numberbeing smaller than +VII thereby forming permanganate. Inorganicbromates, hypochlorites, chlorites, chlorates, peroxodisulfates,monopersulfates as well as the mixtures thereof are proposed asoxidizing agents. The oxidizing agents are reduced during theregenerating reaction and due to this reduction reaction are consumed.

[0005] This method has however proved to be complicated. Moreoveradditional substances are formed in the etching solution due toreplenishment of the oxidizing agents. As a consequence the solutionmust be discarded. For this reason an electrochemical regeneratingmethod has been developped, no undesirable reaction products beingevolved due this method. Such a method ist described in EP 0 291 445 A2.This method is utilized to regenerate permanganate etching solutionsserving to the roughening and cleaning of plastics surfaces, especiallyof printed circuit boards. The method consists of oxidizing manganesehaving an oxidation number of +VI to manganese having an oxidationnumber of +VII by electrochemical oxidation. This reaction is carriedout in an electrochemical device and comprises an anodic reaction. Forthis purpose a cathode and an anode are arranged in an electrochemicaldevice having a diaphragm preferably lying in between thereof. The anodeand the cathode are connected to a current source in such a way that thecathode is polarized cathodically and the anode is polarized anodically.It has been explicitly pointed out in this document that optimumregeneration results are obtained with a device which is provided withtwo electrolyte compartments being separated from one another by adiaphragm. However, sufficient regeneration might also be achieved if adevice would be used which is provided with one electrolyte compartmentonly, into which both electrodes are immersed.

[0006] Furthermore a method is described for etching epoxy resin with analkali permanganate etching solution with an etching rate of more than 3μm, especially in bore holes in printed circuit boards, the etchingsolution containing 10 to 100 g/l alkali permanganate and at least 30g/l alkali hydroxide. The method includes the following method steps: a.stabilizing the permanganate etching solution by electrochemical anodicoxidation at a direct current voltage of from 0.5 to 25 V and at adirect current density of from 0.1 to 20 A/cm²; b. adjusting thepermanganate and OH^(—) ion concentration by electrochemical and/orphotometric measurement and appropriate additional dosage in the eventof a deviation from the desired value.

[0007] A similar application for electrochemically regenerating chemicaloxidizing agents that are used in galvanotechnics has been described byW. P. Innes, W. H. Taller and D. Tomasello in Plat. Surf. Finish., 1978,pages 36-40. This method relates to the regeneration of chromic acidetching solutions. In this case too the anode and the cathode of theregenerating device are separated from one another by a diaphragm, thediaphragm being a long porous ceramic cylinder.

[0008] A further application is disclosed in U.S. Pat. No. 3,470,044.The method described in this document relates to the eletrochemicalregeneration of spent ammonium persulfate etching solutions. Thesesolutions are inter alia utilized to dissolve metals, such as forexample copper, cobalt, iron, nickel, zinc and the alloys thereof. Forthe regeneration reaction the etching solutions are passed through ananode compartment in an electrochemical cell, an anode being arranged inthis compartment. This compartment is separated from a cathodecompartment by a diaphragm this diaphragm being a cathode ion exchangemembrane. A cathode is positioned in the cathode compartment. Duringoperation of the electrochemical device persulfate is produced byoxidation of sulfate at the anode and metal ions are transferred to thecathode compartment the metal ions being reduced to elemental metal atthe cathode.

[0009] A further device for regenerating permanganate etching solutionsis disclosed in JP 6-306668 A. In this case the device comprises aregeneration container, the permanganate etching solution passingtherethrough, further a plurality of cathodes and anodes being arrangedin this container and being opposite to one another. The cathodes arepartly covered by an electrically nonconducting layer, for example apolytetrafluor ethylene layer. For example the cathodes are designed asrods. The rods are only covered with the layer on certain sections.These sections alternate with other sections that are uncovered.

[0010] It has proven that the known methods for etching surfaces ofplastics materials are complicated since either a large amount ofchemicals is consumed for reoxidalizing manganese species having anoxidation number of less than +VII being formed due to the etchingreaction or a large amount of energy is consumed. Especially it hasturned out that a large amount of manganese dioxide sludge is formed inspite of continuous regeneration of the permanganate etching solution.This sludge must be isolated and discarded continuously. Moreover thissludge is blown up in the solution through the hydrogen evolved at thecathode so that this sludge possibly reaches the treatment containerused for etching the plastics parts. Furthermore it has been observedafter a long time of regenerating operation and connected herewith aftera considerable sludge production that roughening of the plasticssurfaces becomes uneven so that the degree of roughening of the plasticssurfaces fluctuates.

[0011] Furthermore it has been observed, especially in cases in whichlarge plastics surfaces per unit time are etched, that the efficiency ofregeneration of permanganate gradually decreases. This behaviorespecially occurs when the SBU technique (sequential build up) isutilized. This technique involves a process in which individual circuitplanes with plastics layers lying in between are manufacturedsequentially. Each dielectric layer in circuit carriers must be cleanedand roughened with a permanganate etching solution in order to assuregood adherence of the subsequent circuit line plane on this dielectriclayer. Consumption of permanganate compounds and formation of therespective degradation products, for example of manganate and manganesedioxide, are considerable per unit time since large areas are to betreated with this process. Up to now a plurality of regenerating deviceshave been arranged in parallel in manufacturing plants in order to keepthe concentration of permanganate in these solutions at the desiredlevel at all. These devices require large floor space so that problemsarise when the individual units are to be arranged side by side(treatment container and regenerating devices). These problems cannot beeasily solved.

[0012] The problem of the present invention therefore comprises avoidingthe disadvantages of known devices and methods and especially comprisesfinding a device being suitable for regeneration of permanganate etchingsolutions with high efficiency.

SUMMARY OF THE INVENTION

[0013] The present invention comprises a novel cathode for anelectrochemical arrangement for regenerating permanganate etchingsolutions.

[0014] The present invention further comprises a novel device forelectrochemically regenerating permanganate etching solutions.

[0015] The present invention also comprises a novel method for themanufacture of a cathode for an arrangement for electrochemicallyregenerating permanganate etching solutions.

[0016] The cathode according to the present invention being utilized inan electrochemical device for regenerating permanganate etchingsolutions is characterized by a porous, electrically nonconducting layeron the surface thereof. This layer is formed in such a way that electriccurrent is able to flow therethrough. The term porous will be understoodin the context of the present invention as being equivalent toperforations. Therefore not only cathodes having conventional porousmaterial as a layer are comprised within the scope of the presentinvention but also cathodes being coated with a fabric material thepores being the interspaces between the meshes of the fabric.

[0017] The electrochemical regenerating device comprises at least oneanode, at least one cathode according to the present invention, acurrent source for the at least one anode and the at least one cathodeas well as electric conducting lines between the current source on theone hand and the at least one anode and the at least one cathode on theother hand.

[0018] The method according to the present invention comprises thefollowing method steps: a. a cathode body is provided with a porous,electrically nonconducting layer, b. further the cathode body beingprovided with the layer is brought into contact with an alkalinepermanganate solution containing manganate, c. an anode is brought intocontact with the alkaline permanganate solution containing manganate andd. by means of an electric current source an electric current isproduced in a current circuit being formed by the cathode and the anodein such a way that an insoluble layer is formed on the surface of thecathode, the layer at least predominantly containing manganese having anoxidation number of +IV.

[0019] When using conventional cathodes and regenerating devices,working up of the etching solutions caused problems since manganesedioxide sludge was produced in excessive quantities, the sludge hardlybeing separable by filtration. Due to the large manganese dioxidegeneration a thick sludge layer formed at the bottom of theelectrochemical cells. Cathodes and anodes partly were immersed intothis layer so that in these regions reoxidation of manganate topermanganate at the anode or a cathodic reaction at the cathodevirtually was no longer possible. Further examination of the mechanismsoccuring at the individual electrodes of conventional regeneratingdevices, which preferably were not equipped with a diaphragm between theanode and the cathode, led to the conclusion that permanganate was notpredominantly reduced to manganate and further to manganese dioxide atthe cathode, if the cathodic current density was sufficiently high. Inthis case the overvoltage at the cathode raised so much thatpredominantly hydrogen was generated at the cathode due to decompositionof water being used as a solvent in the etching solution. Due togeneration of hydrogen the aforementioned electrochemical reaction wasdepressed. Therefore a further decrease of active cathode surface wouldactually had to lead to a further increase of efficiency of theregenerating reaction due to accumulation of manganese dioxide sludge atthe bottom of the regenerating cell since in this case reduction ofpermanganate at the cathode would have been depressed in favor ofhydrogen generation due to a further raise of current density and henceof overvoltage. Surprisingly just the opposite was observed whenconventional devices were utilized, namely a decrease of efficiency whenmanganese dioxide sludge accumulated. The reason for this effect has notbeen found out.

[0020] Furthermore it has turned out that uneven etching withpermanganate etching solutions was attributed to the fact that thecomposition of the etching solutions could no more be held constanteasily due to the high material turnover in the solutions. Frequentlythe etching solutions could not be regenerated completely because anelectrochemical regeneration was only possible if the maximum capacityof the devices being at the disposal for this purpose and being arrangedin parallel was made use of. Therefore under these conditions thepermanganate concentration was below the nominal value and the manganateconcentration was above the respective value.

[0021] Using the cathode and the device according to the presentinvention the problems arising from utilizing conventional devices areeliminated:

[0022] By coating the cathode of the electrochemical regenerating deviceaccording to the present invention with a porous, electricallynonconducting layer on the surface thereof manganese compounds in thepermanganate etching solution having an oxidation number of below +VIIare reoxidized to permanganate with high efficiency. The efficiencybeing defined as the ratio of the electrochemical equivalent of thepermanganate ions formed from manganate ions to the electric chargebeing turned over is improved by a factor of 2 to 5 if the cathodesaccording to the present invention are utilized in the electrochemicalregenerating device. Due to the layer on the cathode surface the areabeing effective for the electrochemical reaction is decreased forexample to the fifth part. However, the local current density raised asa result of this does not lead to the advantageous properties of thecathode according to the present invention. This may be easily seen fromthe fact that reduction of the surface area of the cathode due toreduction of the size of the cathode body does not have the same effectas the layer being deposited on the surface.

[0023] At the same time much less manganese dioxide sludge is generatedas compared to the situation when conventional devices are used.Interestingly it has been observed that manganese dioxide being formedat the cathode according to the present invention sticks fixedly to thecathode surface as a dense coating and does not come off. Already forthis reason much less sludge is generated. Therefore the further problemis also solved that hydrogen being evolved at the cathode ofconventional devices blows up manganese dioxide sludge which could comeinto the treatment container for the plastics parts and there couldcause problems.

[0024] The advantage of the cathodes according to the present inventionand of the regenerating devices which are provided with these cathodeshas especially favorable effects when the SBU technique is utilized inwhich large plastics surface areas are etched per unit time. Now in thiscase due to the increased efficiency of regeneration a smaller number ofelectrochemical regenerating devices according to the present inventionhaving a lower electric total output is sufficient for keeping theetching conditions constant. Therefore in spite of a high materialturnover when plastics surfaces are etched no considerable variationsdue to etching are noticed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The main issue of the present invention relates to a porouselectrically nonconducting layer on the surface of the cathode of anelectrochemical regenerating device.

[0026] The cathode is preferably entirely coated with the layer. Thelayer is designed such that electric current can flow therethrough evenif the cathode is covered entirely with the layer.

[0027] Preferably a layer is used which is resistant to acid and/oralkali, and which is for example made of plastics material or ofceramics. In a preferred embodiment of the present invention the layercomprises and is preferably composed of a fabric especially fittingfirmly to the cathode surface. In this case electric current can flowthrough the layer because gaps are formed between the meshes of thefabric. In another embodiment of the present invention the layer hassmall pores essentially or completely extending through the layer sothat electric conductivity is ensured and is not made of a fabric but ofa relatively dense material completely covering the cathode body.

[0028] The layer may especially comprise and preferably be composed of amaterial, selected from the group comprising polyalkylene andpolyperhalogenalkylene and especially is composed of a material,selected from the group comprising polypropylene (PP), high densitypolyethylene (HDPE) and polytetrafluor ethylene. Especially preferred isa layer essentially consisting of a gauze made from polytetrafluorethylene having a mesh size of from 70 to 100 μm. It is furtherfavorable to use porous HDPE. In principle also porous ceramics may beused. Consequently in the meaning according to the present invention allmaterials in which electric current may flow via channels are understoodas being suitable to form a porous layer.

[0029] Due to the electrochemical reaction of the manganese species atthe cathode having an oxidation number higher than +IV, namely of MnO₄^(—), MnO₄ ² ^(—) and of manganese species with manganese having anoxidation number of +V, an insoluble coating is formed at the cathode,this coating being insoluble and at least predominantly containingmanganese having an oxidation number of +IV and being adsorbed on thecathode surface. This coating together with the porous electricallynonconducting layer forms a joined coating by the manganese dioxidecoating if necessary filling out a gap between the cathode surface andthe layer as well as the pores of the layer and the empty space in thefabric. It will be understood in the present case that manganese dioxideis defined by the reaction products being formed both due toelectrochemical reaction and due to redox reactions taking place inhomogeneous solution or heterogenously. Probably this material ispyrolusite but may also be any of the manganese dioxide minerals. Acathode containing this manganese species is also comprised in the scopeof this invention.

[0030] The composite coating comprises both the porous, insoluble andelectrically nonconducting layer and the coating formed in the gaps andpores of this layer. This composite coating is electrically conductive.This conductivity may be due to pores and gaps in the layer and pores inthe coating but may also be due to pores and gaps in the layer and acertain electric conductivity of the coating itself. Electricconductivity of the coating may for example be due to anonstoichiometric composition of the manganese dioxide which might bedescribed by a formula such as MnO_(2−x), x being a number between 0 and1.

[0031] Therefore it is not necessary to leave the cathode surfacepartially open as is disclosed in JP 6-306668 A. In this case thecathode rods are only partially covered by a layer of an electricallynonconducting material. The main current will in this case be driventhrough those areas at the cathode surface which are free from thisnonconducting layer.

[0032] The cathode body is provided with an electrically conductivesurface and is preferably composed of metal. In an especially preferredembodiment of the invention the cathode body is made from copper or fromspecial stainless steel which is also resistant to hot alkalinepermanganate etching solutions. For example a V4A stainless steel may beused. Stainless steel is preferred over copper since a cathode body madefrom copper would have to be dismantled from the regenerating device onthe occasion of cleaning the regenerating compartment because copperexhibits poor resistance if the material is polarized anodically.

[0033] Depending on the type, the layer may be mounted to the cathodebody or be deposited as a fabricated material, the porous layer thenbeing formed by an appropriate film forming method in this case. Forexample in the former case a tube is first manufactured from the layermaterial which is then mounted to the cathode body. If necessary amaterial is selected for the tube which may be shrunk onto the body. Incase of the latter alternative for example a powder of the material isdeposited on the cathode body and subsequently is sinter-fused thereto,the porous layer being formed during this method. In this way porouslayers may also be manufactured made from ceramics for example.

[0034] Subsequently the cathode body provided with the layer and ananode are brought into contact with an alkaline permanganate solutioncontaining manganate. For this purpose the permanganate solution can beidentical with a typical permanganate etching solution. The conditionsfor generating the manganese dioxide coating may also be identical tothe conditions being applied for etching plastics surfaces. If thecathode body provided with the porous layer is utilized directly in apermanganate etching solution in order to regenerate this solution beingused for etching plastics surfaces the manganese dioxide coating is alsoformed upon starting current flow. An expanded metal part made of steelmay be utilized as an anode as will be the case for regenerating thepermanganate etching solution.

[0035] In order to produce the manganese dioxide coating electriccurrent flow is generated in an electric circuit being formed by thecathode and the anode in such a way that the manganese dioxide coatingis produced at the cathode surface. For this purpose the cathode and theanode are for example supplied with direct current. Preferably aninitially reduced current flow is set. After switching on of anincreased electric current this will be gradually increased until thetypical manganese dioxide layer formation speed is achieved.Surprisingly deposition of manganese dioxide on the cathode surface doesnot proceed during normal operation of the regenerating device.

[0036] Preferably the cathode is formed as a rod, especially being madefrom a metal, preferably from steel. The anode is preferably designed asa perforated cylinder, the cathode being arranged axisymmetrically inthe centre of the anode. Preferably the cylinder as well is composed ofmetal and especially is made from steel. Due to this arrangement an evencurrent density distribution is achieved at the surfaces of the cathodeand of the anode due to a symmetrical electric field in the spacebetween the two electrodes. Because of that the electrochemicalreactions may be set reproducibly at the electrode surfaces so thatdefined conditions are adjusted in the device.

[0037] Especially the anode may be made from expanded metal. Such anodeembodiments may be manufactured very easily. Above all V4A steel may beutilized as material for the anode.

[0038] Preferably the ratio of the geometrical area of the anode to thegeometrical area of the cathode is set to a value of at least 3:1 inorder to minimize generation of manganese dioxide at the cathode. Mostpreferably a ratio of at least 10:1 may be set.

[0039] For the electrochemical regeneration a cathodic current densityin the range of from 25 to 250 A/dm² is set. If a cathode body beingdesigned as a rod, having a diameter of about 15 mm and being immersedinto a permanganate bath for about 45 cm a total current at the cathodewill result of about 50 A to about 500 A.

[0040] The regenerating device comprising the cathode according to thepresent invention and the anode may either be accomodated in a separatecontainer, into which the permanganate etching solution is pumped bymeans of a pumping means via liquid conducting lines and from which thesolution is passed back to the treatment container via other liquidconducting lines, or the regenerating device is integrated into thetreatment container. If sufficient space is available for integratingthe device into the treatment container, the latter alternative is morefavorable, since the treatment solution may get to the regeneratingdevice simply by convection in the container for treatment of theplastics parts. Moreover ideally no additionally pumps and liquidconducting lines are required. Since a considerably higher efficiency ofthe electrochemical reaction is achieved with the regenerating deviceaccording to the present invention than with conventional devices, theformer may be realized in a smaller size than conventional devices.Therefore it is rather possible to arrange the regenerating device inthe treatment container.

[0041] An embodiment not comprising a diaphragm between the cathode andthe anode is more favorable than a corresponding embodiment comprisingsame, since the former is less prone to problems and because a smallerpotential drop und hence a smaller electric power loss in the deviceresults. In principle the regenerating device may also be equipped witha diaphragm separating the cathode compartment from the anodecompartment.

[0042] The regeneration device is preferably utilized in a permanganateetching plant being used to treat printed circuit boards. It has provenespecially favorable if a regenerating device is used in so-calledhorizontal plants in which the printed circuit boards are conveyed in ahorizontal conveying path and brought into contact thereby with apermanganate etching solution. In this case the printed circuit boardsare transported preferably in a horizontal alignment or in a verticalalignment. The advantage of such a combination results from the factthat large plastics surfaces may be treated per unit time, because thethroughput of these horizontal plants is relatively big. Therefore theregenerating device must also have a high efficiency.

[0043] In order to more clearly describe the present invention thefollowing figures are presented, showing preferred embodiments of theinvention:

[0044]FIG. 1: schematic drawing of a anode/cathode couple in aregenerating device;

[0045]FIG. 2: cross section through a cathode according to the presentinvention in a first embodiment;

[0046]FIG. 3: cross section through a cathode according to the presentinvention in a second embodiment.

[0047] A schematic drawing of a regenerating device is shown in FIG. 1.The device may for example be employed in a treatment container forprinted circuit boards.

[0048] The device comprises an anode 1 being made from an expanded metalof V4A steel and a cathode 2 according to the present invention. Theanode 1 has a cylindrical shape. The cathode 2 is also composed of V4Asteel and is designed as a rod. An easy convective passage of treatmentliquid through the perforations of the anode 1 is made possible sincethe anode 1 is made from expanded metal. Therefore liquid iscontinuously exchanged between the anode 1 and the cathode 2 in theinside region of the device and renewed. The cathode rod 2 is arrangedin the region inside the anode 1 in such a way that it is positionedaxisymmetrically the anode cylinder.

[0049] In order to operate the regenerating device the cathode 2 ispolarized cathodically and the anode 1 is polarized anodically.

[0050] A first embodiment of the cathode 2 according to the presentinvention is shown in FIG. 2 in a cross section. The cathode 2 comprisesa cathode body 3 having a layer 7 which in this case is composed of afabric having vertical meshes 4 and horizontal meshes 5. The fabric ismounted to the cathode body 3 as a tube. For example a fabric made frompolytetrafluor ethylene (for example TEFLON® of DuPont de Nemours, Inc.)may be employed. Between the meshes 4,5 a manganese dioxide coating 6 isgenerated through an electrochemical reduction reaction in an alkalinepermanganate solution.

[0051] In order to manufacture this layer the cathode 2 (rod diameterabout 15 mm, depth of immersion into the permanganate solution about 45cm) was brought into contact with a permanganate etching solution havingthe following composition:

[0052] 65 g/l KMnO₄

[0053] 50 g/l NaOH

[0054] in water

[0055] at a temperature of 80° C. the reaction taking place in theregeneration device shown in FIG. 1. Subsequently a voltage was appliedto the electrodes such that an electric current of 5 A flowed betweenthe anode 1 and the cathode 2 (voltage of about 2 V). Due to the currentflowing an initial manganese dioxide coating 6 between the meshes 4,5 ofthe fabric was formed. After about 20 min the current density at thesurface of the cathode 2 was raised to about 10 A (voltage of about 3V). After respective further 20 min a current of about 20 A (voltage ofabout 3.5 V), about 50 A (voltage of about 4.5 V), about 80 A (voltageof about 5 V) and finally about 100 A (voltage of about 5.4 V) wereapplied. The current flow was kept constant thereafter. At the end oftreatment a firmly adsorbed manganese dioxide coating was formed on thecathode body 3.

[0056] If the layer made from the electrically nonconducting fabric isfastened to the cathode body 3 by means of Seeger circlip ringsaccording to German standard DIN 471 or by means of stainless steelclips, the cathode 2 may be formed much quicker in the permanganatesolution using the method indicated. The current decreases to nearlyzero within 1 to 5 sec if too high currents are initially applied duringthe forming procedure, or the voltage rises very steeply so thatinadmissable values for the voltage are achieved (starting from 8 Vstainless steel dissolves electrolytically).

[0057] The forming method described is performed only once with acathode 2. Subsequently this cathode 2 may be utilized in any way toregenerate permanganate solutions. Even after a long operation break thenominal electric current is applicable.

[0058] A further embodiment of a cathode 2 according to the presentinvention is shown in FIG. 3 in a cross section. In this case thecathode 2 is provided with a porous layer 7 of ceramics 8 which firmlyadheres to the cathode body 3.

[0059] A manganese dioxide coating 6 in this case is formed in the poresof the layer of ceramics 8 due to treament of the cathode body 3provided with the layer 7 which has been produced under the sameconditions being given for the cathode 2 in FIG. 2.

[0060] If the embodiments of the cathodes 2 shown in FIG. 2 and FIG. 3are used regenerating devices can be manufactured which have theaforementioned advantageous properties.

[0061] In the following an example and a comparative example are givento more clearly describe the present invention:

EXAMPLE

[0062] In order to manufacture a rod-shaped cathode body having adiameter of 15 mm and made from V4A steel was coated with a single layerof a TEFLON® fabric having a mesh size of 70 μm. The coating was fixedto the cathode body by means of wire rings at spacings of 2 cmrespectively. The diameter of the wire was 0.6 mm. The cathode bodybeing prepared in this way was formed by gradual increase of theelectric current, as described above, a manganese dioxide coating beinggenerated on the cathode body.

[0063] The formed cathode was immersed into a permanganate bath having avolume of 350 I. The bath had been heated to a temperature of 80° C. Thecathode was inserted into an anode with a diameter of 120 mm in aregenerating device as shown in FIG. 1, the anode being mounted in thebath and being formed from a basket made of expanded metal of V4A steel.For this purpose the cathode was fitted to a carrier (not shown) intothe inner compartment of the cylindrical anode in such a way that it waspositioned axisymmetrically to the anode.

[0064] The regenerating device was connected to a direct current sourceand immersed into a treatment container for printed circuit boardscontaining a permanganate etching solution comprising 65 g/l KMnO₄ and50 g/l NaOH in an aqueous solution.

[0065] The current flowing in the regenerating device was set to 100 Aat a voltage of 5.5 V.

[0066] Under the conditions the turnover speed amounted to 2 to 3 gK₂MnO₄ per liter bath and per hour. Manganese dioxide sludge was onlyproduced in slight quantities.

Comparative Example

[0067] The experiment given in the aforementioned example was repeatedusing a conventional cathode in this case having no coating at thesurface. The size of the electrodes, the materials thereof and the otherexperimental conditions were selected to be identical to the size ofelectrode, materials and other experimental conditions given in theaforementioned example. Upon setting an electric current of 100 A avoltage of 5 V was adjusted.

[0068] Under the conditions described the turnover speed amounted onlyto 0.6 g K₂MnO₄ per liter bath and per hour. At the same time largequantities of manganese dioxide sludge were produced.

Reference Numerals

[0069]1 anode

[0070]2 cathode

[0071]3 cathode body

[0072]4 vertical meshes of fabric

[0073]5 horizontal meshes of fabric

[0074]6 manganese dioxide coating

[0075]7 porous, insoluble, electrically nonconducting layer

[0076]8 layer of ceramics

1. Cathode for an electrochemical arrangement for regeneratingpermanganate etching solutions, wherein the cathode (2) is provided witha porous, electrically nonconducting layer (7) on the surface thereof.2. Cathode according to claim 1, wherein the cathode (2) is entirelycoated with the nonconducting layer (7).
 3. Cathode according to any ofthe preceding claims, wherein the layer (7) comprises a plasticsmaterial resistant to acid and/or alkali.
 4. Cathode according to any ofthe preceding claims, wherein the layer (7) comprises a fabric (4,5)fitting firmly to the cathode (2).
 5. Cathode according to any of thepreceding claims, wherein the layer (7) comprises a material, selectedfrom the group comprising polyalkylene und polyperhalogenalkylene. 6.Cathode according to any of the preceding claims, wherein the layer (7)is composed of a material, selected from the group comprisingpolypropylene, high density polyethylene and polytetrafluor ethylene. 7.Cathode according to any of the preceding claims, wherein additionallyan insoluble layer (6) is adsorbed to the surface of the cathode (7),the layer (6) at least predominantly containing manganese having anoxidation number of +IV.
 8. Cathode according to any of the precedingclaims, wherein the cathode (2) is formed as a rod.
 9. Method for themanufacture of a cathode for an arrangement for electrochemicallyregenerating permanganate etching solutions, wherein a. a cathode body(3) is provided with a porous, electrically nonconducting layer (7); b.the cathode body (3) provided with the layer (7) is brought into contactwith an alkaline permanganate solution containing manganate; c. an anode(1) is brought into contact with the alkaline permanganate solutioncontaining manganate; and d. by means of a electric current source anelectric current is produced in a current circuit being formed by thecathode (2) and the anode (1) in such a way that an insoluble layer (6)is formed on the surface of the cathode (2), the layer (6) at leastpredominantly containing manganese having an oxidation number of +IV.10. Method according to claim 9, wherein an initially low current isset, when method step d is performed and that the current is furtherraised gradually.
 11. Device for electrochemically regeneratingpermanganate etching solutions comprising a. at least one anode (1); b.at least one cathode (2) according to any of claims 1 to 8; c. a currentsource for the at least one anode (1) and the at least one cathode (2);as well as d. electric conducting lines between the current source onthe one hand and the at least one anode (1) and the at least one cathode(2) on the other hand.
 12. Device according to claim 11 wherein the atleast one cathode (2) is formed as a rod and the at least one anode (1)is formed as a perforated cylinder, each tcathode (2) being arrangedaxisymmetrically in the centre of at least one anode (1).
 13. Deviceaccording to any of claims 11 and 12, wherein the at least one anode (1)consists of expanded metal.
 14. Device according to any of claims 11 to13, wherein the ratio of the geometrical area of the at least one anode(1) to the geometrical area of the at least one cathode (2) is at least3:1.